In the production of structural components by adhesive bonding of individual components of aluminum or aluminum alloy, it is necessary that the surfaces of these components are pretreated to assure the required bonding strength. In this pretreatment of the aluminum components, the component surfaces are covered with an oxide coating, which is a necessary prerequisite for the desired adhesion stability of the adhesive bond. Until now, it was customary to produce the oxide film on the aluminum components by means of a so-called pickling-process, which works on the basis of chromic acid which provides good pretreatment coatings with strong adhesive properties.
The pickling-process, however, is negatively criticized due to the dangers that it can cause for the environment. Specifically, because the chromate necessary for the pickling should not be discharged into the environment after it has been used. In addition, energy is needed in the pickling process to heat the bath.